A native Google Latitude app for iPhone is soon to be released and should be available in an App Store shortly if it isn't already. Latitude is a service that uses location-based check-ins to let you track your friends online (with all appropriate privacy guards, of course).

UPDATE: Latitude has been pulled from the Japanese App Store. TechCrunch thinks the release might have been accidental and an official, publicized release could be coming soon. Now back to the original post already in progress...

@Mike:
It was updated while I was writing and queueing this, no doubt in the same manner our post was updated while you were writing and posting your incredibly generous, thoughtful, and productive comment :D

I have been using Latitudie for this and frankly, been enjoying the control I have over update intervals and the ability to force gps on in the background for tracking my long distance hikes.
Having used Latitude on Windows Phone and Blackberry devices I am not sure I will switch to it if an official client does not support these functions

Can't believe it's been sooooo long in the making. Had all these features on my Blackberry for a couple of years now.
A step in the right direction Google/Apple.
Also, Latitude already pushes email notifications to you when a friend is closeby.

@zero credibility
Actually it's pretty clever, it only alerts you when a friend is nearby and usually isn't at that point in the day. For example it won't email you everyday to tell you that a friend who lives a mile down the road is at home but it will email you if you're out of the area and a friend is closeby who usually isn't in that location......
Sorry if that sounded confusing! haha

@Omar and @Rodney, totally agree. Maybe it's time to put the phone down, look around you and see that other people exist around you. You might even try talking to one of them. Hint: you do that with words, not text. Sheesh...

I don't get what it is with people being so scared of Latitude... First off, no one can see where you are unless you EXPLICITLY give each individual permission (or you make it public). Second you don't even have to use that feature. You can simply just use it to track your phone's whereabouts or retrace your steps.

To tell you the truth, I'm not sure I'd want\use this service\feature. I guess I just don't care where my friends are - LOL
Now, in the case of someone getting kidnapped or something, then I see the point of it. But wait, what about Apple's 'Find My iPhone' ;)

Firstly, you don't have to use it if you don't want to!
You only accept friends that you wish to share your location with and even then you don't have to share your exact location, you have the option of just showing which city you're in rather than the exact location, you can even hide from specific people as and when you wish.
If people want to stalk you then they will! they're hardly likely to do it by adding you as a friend on Latitude! I really like the idea of seeing where friends and family are, if I finish work late I can see that a couple of my friends are still about or even on the same train home as me and arrange to meet up. Really handy if you're waiting for a family member/friend you can see how far away they are!
Me - "How long you gonna be be?"
Them - "10mins I'm just driving to yours..."
Me - "Rubbish you haven't even left yet..."
JOB DONE! haha